Support obtaining a DefaultCredentials value from JSON data.
Also, add an example, and write more package doc.
For Go 1.9 and higher, rename DefaultCredentials to
Credentials and make the former an alias for the latter.
Updates google/google-api-go-client#247.
Change-Id: I9f9e234ed79f8e08fa13914d9c6c60e0154a06e5
Reviewed-on: https://go-review.googlesource.com/99795
Reviewed-by: Ross Light <light@google.com>
Reviewed-by: Chris Broadfoot <cbro@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Jonathan Amsterdam